Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, a great deal of research and study has been focused on lithium secondary batteries having a high-energy density and a high-discharge voltage. These lithium secondary batteries are also commercially available and widely used.
Further, increased environmental concern has drawn a great deal of intensive research on electric vehicles (EVs) and hybrid electric vehicles (HEVs) which are capable of replacing fossil-fuel driven vehicles such as gasoline vehicles and diesel vehicles, one of the primary causes of air pollution. Although nickel-hydrogen (N1—H2) batteries are largely employed as power sources for EVs and HEVs, numerous studies have been actively made to use lithium secondary batteries having a high-energy density and a high-discharge voltage, consequently with some commercialization outputs.
In general, the lithium-ion secondary battery uses a carbon material as an anode active material and a metal oxide such as lithium cobalt oxide or lithium manganese oxide as a cathode active material and, and is prepared by disposition of a porous polyolefin separator between the anode and cathode and addition of a non-aqueous electrolyte containing a lithium salt such as LiPF6. Upon charging of the battery, lithium ions exit from the cathode active material and migrate to enter into a carbon layer of the anode. In contrast, upon discharging, lithium ions exit from the carbon layer and migrate to enter into the cathode active material. Here, the non-aqueous electrolyte serves as a medium through which lithium ions migrate between the anode and cathode. Such a non-aqueous electrolyte must be stable in a range of operating voltage of the battery and must have an ability to transfer ions at a sufficiently high rate.
The lithium ion-containing non-aqueous electrolyte may be used in the form of a solution where the lithium salt such as LiPF6, readily soluble in the non-aqueous electrolyte, was dissolved in cyclic carbonate solvents. When such a non-aqueous electrolyte uses only a high-polarity cyclic carbonate capable of sufficiently dissociating lithium ions, this may result in problems associated with an increased viscosity of the electrolyte and thus a decreased ionic conductivity.
Therefore, in order to reduce the viscosity of the non-aqueous electrolyte, techniques of using a mixed electrolyte of linear carbonates having a small polarity but a low viscosity are known in the art. Representative examples of such linear carbonates may include dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylmethyl carbonate (EMC). Among these materials, EMC having the lowest freezing point of −55° C. exerts superior low-temperature and life performance when it is used. As examples of the cyclic carbonates, mention may be made of ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC). Inter alia, PC has a low freezing point of −49° C. and thus exerts good low-temperature performance. However, when graphitized carbon having a large volume is used as the anode, PC sharply reacts with the anode during a charging process, and therefore it is difficult to use large amounts of PC. For this reason, EC, which forms a stable protective film at the anode, is primarily used.
As discussed above, in order to reinforce low-temperature performance of the battery, conventional lithium-ion secondary batteries have employed a non-aqueous electrolyte in which LiPF6 salts were mixed in linear or cyclic carbonates and a mixture thereof, by using a low-boiling organic solvent as the organic solvent constituting the non-aqueous electrolyte. The non-aqueous electrolyte having such a composition is known to exhibit the most stable battery properties at both room temperature and high temperature.
However, LiPF6, which is commonly used as the lithium salt in the non-aqueous electrolyte, undergoes lowering of a dissociation degree between Li ions and PF6 anions at a low temperature. Therefore, secondary batteries using LiPF6 as the lithium salt suffer from disadvantages such as a sharp increase of the internal battery resistance and thereby a decreased power output.
On the other hand, when the lithium secondary battery is used in EVs and HEVs, the battery must be operable even under low-temperature conditions such as winter seasons and therefore requires excellent power output properties at a low temperature.
Hence, in order to prevent deterioration of low-temperature performance of the battery while maintaining the constitution of the conventional non-aqueous electrolyte showing stable battery properties at both room temperature and high temperature, research and study toward addition of a separate material to the electrolyte are required. In addition, in order to improve the low-temperature performance of the battery, it is preferred to make an effort toward prevention of increases in the resistance of the battery at a low temperature and prevention of the accompanying deterioration of power output properties.
As to improvement of the low-temperature properties of the secondary battery or capacitor, Japanese Patent Laid-open Publication No. 2001-85058 suggests the use of fluorinated solvents having an asymmetric structure such as HCF2(CF2)3COOEt, HCF2(CF2)3CH2I, FC(CF3)2(CF2)4(CF3)2CF, H(CF2)2OCH3 and the like. However, these solvents suffer from fundamental limitations in application thereof to mass production of the secondary battery, due to very high production costs, and a low dissociation degree of lithium ions, due to a small polarity as compared to carbonate-based solvents.
Therefore, there is also a need in the art for the development of a fabrication technique of the secondary battery which is capable of exerting superior operation performance, simultaneously with realization of low production costs.